78 CONTRIBUTIONS 
gin. The anterior part is sometimes entirely straight from 
the beak to the anterior basal margin. 
P. ellipsis. Plate 3. Fig. 56. 
Description. Shell transversely elliptical, compressed, 
inequilateral, concentrically and minutely striate, with 
small longitudinal rugose ribs, more strongly marked on 
the posterior and anterior parts ; beaks rather elevated, re- 
curved ; hinge teeth very small, numerous and scarcely 
interrupted in the middle ; cicatrices impressed ; cavity of 
the shell shallow ; margin entire. 
Diam. .2, Length 7-20ths, Breadth 9-20ths, of an inch. 
Observations. In some of its characters the ellipsis close- 
ly resembles the minor, but is eminently distinguished by 
the absence of marginal crenulations. It is more transverse 
and has smaller dorsal teeth. Immediately under the beak 
is a fosset-like impression, which may have been the seat 
of the ligament. In this it has some resemblance to the 
nucula; but is placed above the arched row of teeth. 
P. obliqua. Plate 3. Fig. 57. 
Description. Shell oblique, somewhat inflated, very in- 
equilateral, straight at dorsal margin, with minute longitu- 
dinal ribs, concentrically folded ; folds beautifully sculp- 
tured ; beaks rather elevated, pointed ; hinge teeth few, 
small, placed on nearly a straight plate, smaller in the 
middle ; cicatrices slightly impressed ; cavity of the shell 
